Thermosetting resin compositions useful as underfill sealants

ABSTRACT

The present invention provides a thermosetting resin composition useful as an underfilling sealant composition which rapidly fills the underfill space in a semiconductor device, such as a flip chip assembly which includes a semiconductor chip mounted on a carrier substrate, enables the semi-conductor to be securely connected to a circuit board by short-time heat curing and with good productivity, and demonstrates acceptable heat shock properties (or thermal cycle properties). The thermosetting resin composition which is used as an underfilling sealant between such a semiconductor device and a circuit board to which the semiconductor device is electrically connected, includes an epoxy resin component and a latent hardener component. The latent hardener component includes a cyanate ester component and an imidizole component.

This application claims benefit to Ser. No. 60/053,592 filed Jul. 24,1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to thermosetting resin compositions useful asunderfill sealants for mounting under a circuit board direct chip attach(“DCA”) packages, which have a semiconductor chip on a carriersubstrate.

2. Brief Description of Related Technology

In recent years, the popularity of smaller-sized electronic applianceshas made desirable size reduction of semiconductor devices. As a result,chip packages are becoming reduced in size to substantially that of thebare die themselves. Such smaller-sized chip packages improve thecharacteristics of the microelectronic device in which it is used, whileretaining many beneficial operating features. This serves to protectsemiconductor bare chips, and increases their reliability and usefullife.

Ordinarily, chip assemblies are connected to electrical conductors on acircuit board by use of solder connection or the like. However, when theresulting chip/circuit board structure is subjected to conditions ofthermal cycling, reliability becomes suspect due to fatigue of thesolder connection between the circuit board and the chip assembly.Recent manufacturing advances provide a sealing resin (often referred toas underfill sealant) in the space created by the mounting of a DCA[such as a chip scale package (“CSP”)/ball grid array (“BGA”) assembly]onto a circuit board to relieve stresses caused by thermal cycling.Underfill sealants have been seen to improve heat shock properties andenhance the reliability of such structures.

Of course, curable resin compositions generally are known. See e.g.,U.S. Pat. No. 4,645,803 (Kohli) which relates to curable epoxy resincompositions of reinforcing filaments and epoxy resins together with aprimary amine-funtional curing agent with or without a polyamine curingagent and a curing catalyst which when cured into a fiber matrix isuseful in preparing composites and prepreg materials for structuralapplications.

In addition, U.S. Patent No. 4,499,245 (Ikeguchi) relates to a curableresin composition requiring a mixture and/or a reaction product of (a) apolyfunctional cyanate ester, prepolymer of the cyanate ester orcoprepolymer of the cyanate ester and an amine and (b) a polyhydantoinresin—a phenolic-based epoxy curative. In addition, a polyfunctionalmaleimide, prepolymer of the maleimide or coprepolymer of the maleimideand an amine may be included as a component (c). These compositions arereported to be useful as coating materials for rust prevention, flameresistance, flame retardation; electrical insulation varnish; andlaminates for use with furniture, building materials, and sheathingmaterials.

And more specifically thermosetting compositions of cyanate esters andepoxy resins are also generally known. See e.g., Japanese patentdocument JP 62-275,123, an English-language abstract of which speaks toa resin composition for preparing prepreg materials with reinforcingfiber for structural applications. The compositions are reported toinclude certain cyanate esters, bismaleimide, polyether sulfone (as anon-reactive thermoplast whose use is as a toughening agent) andbisphenol F- or A-type epoxy resin. In addition, the composition isreported to be optionally hardened by a hardening catalyst, one of whichis noted as imidazole.

U.S. Pat. No. 4,918,157 (Gardner) relates to the use of urea compoundsas latent cure accelerators for cyanate esters, and to thermosettingcyanate ester formulations of cyanate esters and urea compounds. Morespecifically, the '157 patent claims a thermosetting composition of acyanate ester; a urea compound selected from alkyl aryl ureas, arylureas and mixtures thereof; and an epoxy resin. The curable cyanateester formulations of the '157 patent are reportedly useful as matrixresins, and for the production of prepreg, fiber-reinforced laminates,composites and the like.

Epoxy curing systems are also known. See e.g., U.S. Pat. No. 3,862,260(Sellers), in which a curing agent of a trifunctional hardener (such asthe reaction product of one mole of bisphenol A with one mole offormaldehyde) and an imidazole is disclosed.

These uses for thermosetting resin compositions appear to be directed tostructural applications, as contrasted to the microelectronicapplication to which the compositions of the present invention aredirected. To that end, the use of epoxy resin compositions as matrixcompositions for fiber reinforcement in prepreg, composite and laminatematerials for structural materials differs significantly from the use ofepoxy resin compositions as an adhesive and encapsulant inmicroelectronic applications, such as with electrical solder junctionsin semiconductor chips, and creates different demands and requirementsfrom the uncured resin as well as cured reaction products thereof.

A drawback to resin compositions presently used in microelectronicsapplications, such as for underfill sealants, is their extended cureschedule. In addition, providing such resins with a commerciallyacceptable useful working life at room temperature or dispensingtemperatures has been problematic.

Generally, at temperatures near room temperature, the resins begin tocure upon introduction of the curing agent or catalyst. This causesviscosity increases which leads to reduced dispensability. While suchviscosity increase may be alleviated to some degree by using a liquidcuring agent or catalyst, liquid catalysts tend to decrease latency to apoint which is not commercially practical with current productiondemands. And introduction of a solid catalyst, such as imidazole, haslimited applicability because its presence often changes the rheologicalproperties of the composition, and decreases flow.

Thus, due at least in part to their extended cure schedules and limiteduseful working life, manufacturing capacity of certain microelectronicproduction lines has been hampered.

While seemingly simple, the solution to the problem of enhancing thecure speed of recently-used underfill sealants has ordinarily adverselyimpacted their useful working life. Thus, in the event thatpresently-used underfill sealants could be rendered more reactive, theiruseful working life may be further decreased, thereby removing theincentive to prepare a more reactive thermosetting resin composition forunderfill sealing.

Accordingly, it would be desirable for an underfilling sealantcomposition to provide good adhesive properties while flowing and curingin a sufficiently quick time to be commercially appealing and possessingan extended useful working life.

SUMMARY OF THE INVENTION

The present invention provides a thermosetting resin composition usefulas an underfilling sealant composition which (1) rapidly fills theunderfill space in a semiconductor device, such as a flip chip assemblywhich includes a semiconductor chip mounted on a carrier substrate, (2)enables the semiconductor to be securely connected to a circuit board byshort-time heat curing and with good productivity, and (3) demonstratesexcellent heat shock properties (or thermal cycle properties).

The thermosetting resin compositions of this invention which are used asunderfill sealants between such a semiconductor device and a circuitboard to which the semiconductor device is electrically connected,includes an epoxy resin component and a latent hardener component. Thelatent hardener component includes a cyanate ester component and animidazole component.

By using the thermosetting resin compositions of this invention,semiconductor devices, such as flip chip assemblies, may be (1)assembled quickly and without production line down time because ofimproved cure speed and extended useful working life, and (2) securelyconnected to a circuit board by short-time heat curing of thecomposition, with the resulting mounted structure (at least in part dueto the cured composition) demonstrating excellent heat shock properties(or thermal cycle properties).

The compositions of this invention may also be used for microelectronicapplications beyond sealing underfill, such as with glob top, dieattachment and other applications for thermosetting compositions inwhich rapid cure time and an extended useful working life are desirable.

The benefits and advantages of the present invention will become morereadily apparent after a reading of the “Detailed Description of theInvention” together with the figure.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 depicts a cross-sectional view showing an example of a mountedstructure with which the thermosetting resin composition of the presentinvention is used as an underfill sealant.

DETAILED DESCRIPTION OF THE INVENTION

As noted above, the thermosetting resin compositions which are useful asunderfill sealants between a semiconductor device and a circuit board towhich the semiconductor device is electrically connected, include anepoxy resin component and a latent hardener component. The latenthardener component includes a cyanate ester component and an imidazolecomponent.

Typically, the composition includes about 100 parts by weight of theepoxy resin component, and 0 to about 30 parts by weight of the latenthardener component, of which 0 to about 15 parts is comprised of thecyanate ester component and 0 to about 15 parts is comprised of theimidazole component. Desirably, the latent hardener component shouldinclude about 4 parts each of the cyanate ester component and theimidazole component.

The epoxy resin component of the present invention may include anycommon epoxy resin. This epoxy resin may be comprised of at least onemultifunctional epoxy resin, optionally, together with at least onemonofunctional epoxy resin. Ordinarily, the multifunctional epoxy resinshould be included in amount within the range of about 20 parts to about100 parts of the epoxy resin component. In the case of bisphenol F-typeepoxy resin, desirably the amount thereof should be in the range of fromabout 40 to 80 parts.

A monofunctional epoxy resin, if present, should ordinarily be used as areactive diluent, or crosslink density modifier. In the event such amonofunctional epoxy resin is included as a portion of the epoxy resincomponent, such resin should be employed in an amount of up to about 20%by weight based on the total epoxy resin component.

The monofunctional epoxy resin should have an epoxy group with an alkylgroup of about 6 to about 28 carbon atoms, examples of which includeC₆-C₂₈ alkyl glycidyl ethers, C₆-C₂₈ fatty acid glycidyl esters andC₆-C₂₈ alkylphenol glycidyl ethers.

Such epoxy resin(s) include generally, but are not limited to,polyglycidyl ethers of polyvalent phenols, for example pyrocatechol;resorcinol; hydroquinone; 4,4′-dihydroxydiphenyl methane4,4′-dihydroxy-3,3′-dimethyldiphenyl methane; 4,4′-dihydroxydiphenyldimethyl methane; 4,4′-dihydroxydiphenyl methyl methane;4,4′-dihydroxydiphenyl cyclohexane; 4,4′-dihydroxy-3,3′-dimethyldiphenylpropane; 4,4′-dihydroxydiphenyl sulfone; tris(4-hydroxyphyenyl)methane;polyglycidyl ethers of the chlorination and bromination products of theabove-mentioned diphenols; polyglycidyl ethers of novolacs (i.e.,reaction products of monohydric or polyhydric phenols with aldehydes,formaldehyde in particular, in the presence of acid catalyst;polyglycidyl ethers of diphenols obtained by esterifying 2 moles of theethers of diphenols obtained by esterifying 2 moles of the sodium saltof an aromatic hydrocarboxylic acid with 1 mole of a dihaloalkane ordihalogen dialkyl ether (see U.K. Pat. No. 1,017,612, the disclosure ofwhich is hereby expressly incorporated herein by reference); andpolyglycidyl ethers of polyphenols obtained by condensing phenols andlong-chain halogen paraffins containing at least two halogen atoms (seeU.K. Pat. No. 1,024,288, the disclosure of which is hereby expresslyincorporated herein by reference).

Other suitable epoxy compounds include polyepoxy compounds based onaromatic amines and epichlorohydrin, such as N,N′-diglycidyl-aniline;N,N′-dimethyl-N,N′-diglycidyl-4,4′-diaminodiphenyl methane;N,N,N′,N′-tetraglycidyl-4,4′-diaminodiphenyl methane;N-diglycidyl-4-aminophenyl glycidyl ether; andN,N,N′,N′-tetraglycidyl-1,3-propylene bis-4-aminobenzoate.

Examples of the multifunctional epoxy resin include bisphenol A-typeepoxy resin, bisphenol F-type epoxy resin, phenol novolac-type epoxyresin, and cresol novolac-type epoxy resin.

Among the epoxy resins suitable for use herein are polyglycidylderivatives of phenolic compounds, such as those available commerciallyunder the tradenames EPON 828, EPON 1001, EPON 1009, and EPON 1031, fromShell Chemical Co.; DER 331, DER 332, DER 334, and DER 542 from DowChemical Co.; and BREN-S from Nippon Kayaku, Japan. Other suitable epoxyresins include polyepoxides prepared from polyols and the like andpolyglycidyl derivatives of phenol-formaldehyde novolacs, the latter ofwhich are available commercially under the tradenames DEN 431, DEN 438,and DEN 439 from Dow Chemical Company. Cresol analogs are also availablecommercially ECN 1235, ECN 1273, and ECN 1299 from Ciba-GeigyCorporation. SU-8 is a bisphenol A-type epoxy novolac available fromInterez, Inc. Polyglycidyl adducts of amines, aminoalcohols andpolycarboxylic acids are also useful in this invention, commerciallyavailable resins of which include GLYAMINE 135, GLYAMINE 125, andGLYAMINE 115 from F.I.C. Corporation; ARALDITE MY-720, ARALDITE 0500,and ARALDITE 0510 from Ciba-Geigy Corporation and PGA-X and PGA-C fromthe Sherwin-Williams Co.

And of course combinations of the different epoxy resins are alsodesirable for use herein.

In choosing epoxy resins for the epoxy resin component of thecompositions of the present invention, consideration should also begiven to viscosity and other properties thereof.

The cyanate esters useful as a component in the latent hardeningcomponent may be chosen from aryl compounds having at least one cyanateester group on each molecule and may be generally represented by theformula Ar(OCN)_(m), where m is an integer from 2 to 5 and Ar is anaromatic radical. The aromatic radical Ar should contain at least 6carbon atoms, and may be derived, for example, from aromatichydrocarbons, such as benzene, biphenyl, naphthalene, anthracene, pyreneor the like. The aromatic radical Ar may also be derived from apolynuclear aromatic hydrocarbon in which at least two aromatic ringsare attached to each other through a bridging group. Also included arearomatic radicals derived from novolac-type phenolic resins—i.e.,cyanate esters of these phenolic resins. The aromatic radical Ar mayalso contain further ring-attached, non-reactive substituents.

Examples of such cyanate esters include, for instance,1,3-dicyanatobenzene; 1,4-dicyanatobenzene; 1,3,5-tricyanatobenzene;1,3-, 1,4-, 1,6-, 1,8-, 2,6- or 2,7-dicyanatonaphthalene;1,3,6-tricyanatonaphthalene; 4,4′-dicyanato-biphenyl;bis(4-cyanatophenyl)methane and 3,3′,5,5′-tetramethylbis(4-cyanatophenyl)methane;2,2-bis(3,5-dichloro-4-cyanatophenyl)propane;2,2-bis(3,5-dibromo-4-dicyanatophenyl)propane;bis(4-cyanatophenyl)ether; bis(4-cyanatophenyl)sulfide;2,2-bis(4-cyanatophenyl)propane; tris(4-cyanatophenyl)-phosphite;tris(4-cyanatophenyl)phosphate; bis(3-chloro-4-cyanatophenyl)methane;cyanated novolac; 1,3-bis[4-cyanatophenyl-1-(methylethylidene)]benzeneand cyanated bisphenol-terminated polycarbonate or other thermoplasticoligomer.

Other cyanate esters include cyanates disclosed in U.S. Pat. Nos.4,477,629 and 4,528,366, the disclosure of each of which is herebyexpressly incorporated herein by reference; the cyanate esters disclosedin U.K. Pat. No. 1,305,702, and the cyanate esters disclosed inInternational Patent Publication WO 85/02184, the disclosure of each ofwhich is hereby expressly incorporated herein by reference. Of course,combinations of these cyanate esters within the imidizole component ofthe compositions of the present invention are also desirably employedherein.

A particularly desirable cyanate ester for use herein is availablecommercially from Ciba Geigy Corporation, Tarrytown, N.Y. under thetradename AROCY 366(1,3-bis[4-cyanatophenyl-1-(methylethylidene)]benzene).

The imidazole component of the latent hardener component includeimidazoles, such as imidazole and derivatives thereof, such asisoimidazole, imidazole, alkyl substituted imidazoles, such as 2-methylimidazole, 2-ethyl-4-methylimidazole, 2,4-dimethylimidazole,butylimidazole, 2-heptadecenyl-4-methylimidazole, 2-methylimidazole,2-undecenylimidazole, 1-vinyl-2-methylimidazole,2-n-heptadecylimidazole, 2-undecylimidazole, 2-heptadecylimidazole,2-phenylimidazole, 2-ethyl 4-methylimidazole,1-benzyl-2-methylimidazole, 1-propyl-2-methylimidazole,1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole,1-cyanoethyl-2-undecylimidazole, 1-cyanoethyl-2-phenylimidazole,1-guanaminoethyl-2-methylimidazole and addition products of an imidazolemethylimidazole and addition products of an imidazole and trimelliticacid, 2-n-heptadecyl-4-methylimidazole and the like, generally whereeach alkyl substituent contains up to about 17 carbon atoms anddesirably up to about 6 carbon atoms; aryl substituted imidazoles, suchas phenylimidazole, benzylimidazole, 2-methyl-4,5-diphenylimidazole,2,3,5-triphenylimidazole, 2-styrylimidazole, 1-(dodecylbenzyl)-2-methylimidazole,2-(2-hydroxyl-4-t-butylphenyl)-4,5-diphenylimidazole,2-(2-methoxyphenyl)-4,5-diphenylimidazole,2-(3-hydroxyphenyl)-4-,5-diphenylimidazole,2-(p-dimethylaminophenyl)-4,5-diphenylimidazole,2-(2-hydroxyphenyl)-4,5-diphenylimidazole,di(4,5-diphenyl-2-imidazole)-benzene-1,4,2-naphthyl-4,5-diphenylimidazole, 1-benzyl-2-methylimidazole,2-p-methoxystyrylimidazole, and the like generally where each arylsubstituent contains up to about 10 carbon atoms and desirably up toabout 8 carbon atoms.

Of course, combinations of these imidazoles are also desirable as theimidazole component of the latent hardener component of the compositionsof the present invention.

The thermosetting resin compositions of the present invention may be ofthe one-pack type, in which all the ingredients are mixed together, orof the two-pack type in which the epoxy resin component and latenthardener component are stored separately and mixed together prior touse.

During application, the thermosetting resin compositions according tothe present invention penetrate and flow readily into the space betweenthe circuit board and the semiconductor device, or at least show areduction in viscosity under heated or use conditions thus penetratingand flowing easily.

Generally, it is desirable to prepare the thermosetting resincompositions of this invention by selecting the types and proportions ofvarious so that the gel times will be tailored to a specified period oftime (such as 1 minute or 2 minutes) at a temperature of about 150° C.In such case, the inventive compositions should show no or substantiallyno increase of viscosity after a period of time of about six hours. Withsuch a gel time, the compositions penetrate into the space between thecircuit board and the semiconductor device (e.g., of 100 to 200 μm)relatively rapidly, and allow for a greater number of assemblies to befilled without observing a viscosity increase in the composition therebyrendering it less effective for application.

Optionally, the thermosetting resin composition of the present inventionmay further contain other additives such as defoaming agents, levelingagents, dyes, pigments and fillers. Moreover, the compositions may alsocontain photopolymerization initiators, provided such materials do notadversely affect the desired properties of the composition.

In an additional aspect of this invention, there is provided filledthermosetting compositions. These compositions, in addition to the epoxyresin component and latent hardener component, include a fillercomponent. The filler component acts to lower moisture pick up, andtends to increase viscosity. Appropriate filler components includesilica, alumina, silica-coated aluminum nitride, silver flake and thelike.

Generally, about 0.1 to about 300 parts of the filler component may beused, with about 150 to 180 parts being desirable.

Reference to FIG. 1 shows a mounted structure (i.e., a flip chippackage) in which a thermosetting resin composition of the presentinvention has been applied and cured.

The flip chip package 4 is formed by connecting a semiconductor chip (abare chip) 2 to a carrier substrate 1 (e.g., a circuit board) andsealing the space therebetween suitably with a thermosetting resincomposition 3. This semiconductor device is mounted at a predeterminedposition on the carrier substrate 1, and electrodes 5 and 6 areelectrically connected by a suitable electrical connection means 7 and8, such as solder. In order to improve reliability, the space betweenthe semiconductor chip 2 and the carrier substrate 1 is sealed with athermosetting resin composition 3, and then cured. The cured product ofthe thermosetting resin composition should completely fill that space.

Carrier substrates may be constructed from ceramic substrates of Al₂O₃,SiN₃ and mullite (Al₂O₃—SiO₂); substrates or tapes of heat-resistantresins, such as polyimides; glass-reinforced epoxy; ABS and phenolicsubstrates which are also used commonly as circuit boards; and the like.Any electrical connection of the semiconductor chip to the carriersubstrate may be used, such as connection by a high-melting solder orelectrically (or anisotropically) conductive adhesive, wire bonding, andthe like. In order to facilitate connections, the electrodes may beformed as bumps.

In a typical mounting process, solder ball (e.g., in cream or form) maybe printed at appropriate positions on a carrier substrate and suitablydried to expel solvent. A semiconductor chip may then mounted inconformity with the pattern on the carrier substrate. This carriersubstrate is then passed through a reflowing furnace to melt the solderto connect the semiconductor chip. Moreover, the solder may be appliedor formed on either the carrier substrate or the semiconductor chip.Alternatively, this connection may also be made by an electricallyconductive adhesive or an anisotropically conductive adhesive.

After the semiconductor chip is electrically connected to the carriersubstrate, the resulting structure is ordinarily subjected to acontinuity test or the like. After passing such test, the semiconductorchip may be fixed thereto with a thermosetting resin composition, asdescribed below. In this way, in the event of a failure, thesemiconductor chip may be removed before it is fixed to the carriersubstrate with the thermosetting resin composition.

Using a suitable application means, such as a dispenser, a thermosettingresin composition in accordance with this invention is applied to theperiphery of the electronically-connected semiconductor chip. Thecomposition penetrates by capillary action into the space between thecarrier substrate and the semiconductor chip.

The thermosetting resin composition is then thermally cured by theapplication of heat. During the early stage of this heating, thethermosetting resin composition shows a significant reduction inviscosity and hence an increase in fluidity, so that it more easilypenetrates into the space between the carrier substrate and thesemiconductor chip. Moreover, by preheating the carrier substrate, thethermosetting resin composition is allowed to penetrate fully into theentire space between the carrier substrate and the semiconductor chip.

Cured reaction products of the thermosetting resin compositions of thepresent invention demonstrate excellent adhesive force, heat resistanceand electric properties, and acceptable mechanical properties, such asflex-cracking resistance, chemical resistance, moisture resistance andthe like, for the applications for which they are used herein.

The amount of thermosetting resin composition applied should be suitablyadjusted so as to fill almost completely the space between the carriersubstrate and the semiconductor chip, which amount of course may varydepending on application.

Thermosetting resin compositions of the present invention may ordinarilybe cured by heating to a temperature in the range of about 120 to about180° C. for a period of time of about 0.5 to 30 minutes. However,generally after application of the composition, an initial cure time ofabout 1 minute sets up the composition, and complete cure is observedafter about 15 minutes at 150° C. Thus, the composition of the presentinvention can be used in relatively moderate temperatures and short-timecuring conditions, and hence achieve very good productivity.

The present invention will be more readily appreciated with reference tothe examples which follow.

EXAMPLES

In these examples, compositions in accordance with the present inventionwere prepared and evaluated for performance in contrast withcompositions prepared without the cyanate ester component of the latenthardener component. Results are set forth below.

Example 1

Thermosetting Resin Composition—1 Minute Gel Time

A. A thermosetting resin composition for underfill applications inaccordance with the present invention was prepared by mixing togetherwith stirring for a period of time of about 10 minutes at roomtemperature in an open vessel an epoxy resin component including 92parts by weight of bisphenol F-type epoxy resin, and a latent hardenercomponent including 4 parts by weight of 2-ethyl-4-methylimidazole, and4 parts by weight of1,3-bis[4-cyanatophenyl-1-(methylethylidene)]benzene as a cyanate esterresin (commercially available under the tradename AROCY 366 fromCiba-Geigy).

While the composition was used upon formation, it may be stored for aperiod of time of up to about 3 to about 6 months at a temperature ofabout −20° C. without experiencing viscosity increase.

After formation, the composition was transferred to a 10 ml syringe madeof non-reactive plastic, and the composition was dispensed through the12 G needle of the syringe into the junction between the carriersubstrate and semiconductor chip in a previously-formed assembly. Assuch, the composition acts as an encapulant for the electrical solderconnection.

After dispensing was complete, the assembly was transferred to an ovenwhile the temperature was maintained at about 150° C. The compositioncured initially after about 1 minute, and thereafter cured completelyafter about 15 minutes at that temperature.

Separately, the composition was placed between a pair of lap shears andcured in the same way as the assembly above. The bound lap shears wereremoved from the oven and allowed to reach room temperature, at whichpoint they were evaluated for bond strength. The cured composition wasfound to possess lap shear strength of about 1660 psi—which isacceptable for the microelectronic applications which the compositionsare designed.

With respect to shelf-life stability, as noted above the gel time of thecomposition was tailored to 1 minute at a temperature of 150° C. Thiscomposition was observed to experience no viscosity increase at roomtemperature after a period of time of 6 hours; after a period of time ofabout 15 hours the viscosity increase was observed to be about 52%; andafter a period of time of about 24 hours the viscosity increase wasobserved to be about 88%.

B. A comparable composition was prepared in which no cyanate ester wasadded. That composition included 96.5 parts bisphenol F-type epoxy resinand 3.5 parts of 2-ethyl-4-methylimidazole. The composition was alsotailored to a gel time of about 1 minute at a temperature of 150° C. Thelap shear strength was observed to be about 1670 psi.

This composition was applied as above, and demonstrated a viscosityincrease at room temperature after a period of time of about 6 hours ofabout 13%; after a period time of about 15 hours of about 153% and aftera period of time of 24 hours of about 267%.

Accordingly, it is seen that the presence of the cyanate ester componentin the inventive compositions has a dramatic affect in maintaining auseful working life for the compositions with respect to slowingviscosity increase over time at room temperature.

Example 2

Thermosetting Resin Composition—2 Minute Gel Time

A. A thermosetting resin composition for underfill applications inaccordance with the present invention was prepared by mixing togetherwith stirring for a period of time of about 10 minutes at roomtemperature in an open vessel an epoxy resin component including 88parts by weight of bisphenol F-type epoxy resin, and a latent hardenercomponent including 8 parts by weight of 2-ethyl-4-methylimidazole, and4 parts by weight of1,3-bis[4-cyanatophenyl-1-(methylethylidene)]benzene as a cyanate esterresin (AROCY 366).

While this composition was used upon formation, it too may be stored fora period of time of up to about 3 to about 6 months at a temperature ofabout −20° C. without experiencing viscosity increase.

After formation, the composition was transferred to a 10 ml syringe madeof non-reactive plastic, and the composition was dispensed through the12 G needle of the syringe into the junction between the carriersubstrate and semiconductor chip in a previously-formed assembly to actas an encapulant for the electrical solder connection.

After dispensing was complete, the assembly was transferred to an ovenwhile the temperature was again maintained at about 150° C. Thecomposition cured initially after about 2 minutes, and thereafter curedcompletely after about 15 minutes at that temperature.

This composition was also placed between a pair of lap shears and curedin the same way as the assembly above, and as in Example 1. The lapshears were removed from the oven and allowed to reach room temperature,at which point they were evaluated for bond strength. The curedcomposition was found to possess lap shear strength of about 1620psi—which is acceptable for the microelectronic applications for whichthe compositions are designed.

With respect to shelf-life stability, as noted above the gel time of thecomposition was tailored to 2 minute at a temperature of 150° C. Thiscomposition was observed to experience a viscosity increase at roomtemperature after a period of time of 6 hours of about 3%; after aperiod of time of about 15 hours the viscosity increase was observed tobe about 10%; and after a period of time of about 24 hours the viscosityincrease was observed to be about 20%.

B. A comparable composition was prepared in which no cyanate ester wasadded. That composition included 97.5 parts bisphenol F-type epoxy resinand 2.5 parts of 2-ethyl-4-methylimidazole. The composition was alsotailored to a gel time of about 2 minutes at a temperature of 150° C.The lap shear strength was observed to be about 1600 psi.

This composition was applied as above, and demonstrated a viscosityincrease at room temperature after a period of time of about 6 hours ofabout 16%; after a period time of about 15 hours of about 82% and aftera period of time of 24 hours of about 162%.

Accordingly, it is seen that the presence of the cyanate ester in theinventive compositions has a dramatic affect in maintaining a usefulworking life for the compositions with respect to slowing viscosityincrease over time at room temperature.

What is claimed is:
 1. A thermosetting resin composition capable ofsealing underfilling between a semiconductor device and a circuit boardto which said semiconductor device is electrically connected, saidcomposition comprising: (a) about 100 parts of an epoxy resin component;and (b) a latent hardener component in an amount up to about 30 partscomprising (i) a cyanate ester component in an amount up to about 15parts, and (ii) an imidazole component in an amount up to about 15parts.
 2. The composition of claim 1, wherein said epoxy resin comprisesat least one multifunctional epoxy resin.
 3. The composition accordingto claim 1, wherein the epoxy resin component includes members selectedfrom the group consisting of C₆-C₂₈ alkyl glycidyl ethers; C₆-C₂₈ fattyacid glycidyl esters; C₆-C₂₈ alkylphenol glycidyl ethers; polyglycidylethers of pyrocatechol, resorcinol, hydroquinone, 4,4′-dihydroxydiphenylmethane, 4,4′-dihydroxy-3,3′-dimethyldiphenyl methane,4,4′-dihydroxydiphenyl dimethyl methane, 4,4′-dihydroxydiphenyl methylmethane, 4,4′-dihydroxydiphenyl cyclohexane,4,4′-dihydroxy-3,3′-dimethyldiphenyl propane, 4,4′dihydroxydiphenylsulfone, and tris(4-hydroxyphyenyl)methane; polyglycidyl ethers of thechlorination and bromination products of the above-mentioned diphenols;polyglycidyl ethers of novolacs; polyglycidyl ethers of diphenolsobtained by esterifying ethers of diphenols obtained by esterifyingsalts of an aromatic hydrocarboxylic acid with a dihaloalkane ordihalogen dialkyl ether; polyglycidyl ethers of polyphenols obtained bycondensing phenols and long-chain halogen paraffins containing at leasttwo halogen atoms; N,N′-diglycidyl-aniline;N,N′-dimethyl-N,N′-diglycidyl-4,4′diaminodiphenyl methane;N,N,N′,N′-tetraglycidyl-4,4′diaminodiphenyl methane;N-diglycidyl-4-aminophenyl glycidyl ether;N,N,N′,N′-tetraglycidyl-1,3-propylene bis-4-aminobenzoate; bisphenol Aepoxy resin, bisphenol F epoxy resin, phenol novolac epoxy resin, andcresol novolac epoxy resin.
 4. The composition according to claim 1,wherein the epoxy resin component includes bisphenol F epoxy resin andepoxy cresol novalac resin.
 5. The composition according to claim 1,wherein the epoxy resin component includes about 80 parts bisphenol Fepoxy resin and about 20 parts epoxy cresol novalac resin.
 6. Thecomposition according to claim 1, wherein the cyanate ester component ischosen from aryl compounds having at least one cyanate ester group oneach molecule.
 7. The composition according to claim 1, wherein thecyanate ester component is represented by Ar(OCN)_(m), wherein Ar is anaromatic radical and m is an integer from 2 to
 5. 8. The compositionaccording to claim 1, wherein the cyanate ester component is selectedfrom the group consisting of 1,3-dicyanatobenzene; 1,4-dicyanatobenzene;1,3,5-tricyanatobenzene; 1,3-, 1,4-, 1,6-, 1,8-, 2,6- or2,7-dicyanatonaphthalene; 1,3,6-tricyanatonaphthalene;4,4′-dicyanato-biphenyl; bis(4-cyanatophenyl)methane and3,3′,5,5′-tetramethyl bis(4-cyanatophenyl)methane;2,2-bis(3,5-dichloro-4-cyanatophenyl)propane;2,2-bis(3,5-dibromo-4-dicyanatophenyl)propane;bis(4-cyanatophenyl)ether; bis(4-cyanatophenyl)sulfide;2,2-bis(4-cyanatophenyl)propane; tris(4-cyanatophenyl)-phosphite;tris(4-cyanatophenyl)phosphate; bis(3-chloro-4-cyanatophenyl)methane;cyanated novolac; 1,3-bis[4-cyanatophenyl-1-(methylethylidene)]benzeneand cyanated bisphenol-terminated polycarbonate or other thermoplasticoligomer.
 9. The composition according to claim 1, wherein the cyanateester component is 1,3-bis[4-cyanatophenyl-1-(methylethylidene)]benzene.10. The composition according to claim 1, wherein the imidazolecomponent is a member selected from the group consisting of imidazole,isoimidazole, 2-methyl imidazole, 2-ethyl-4-methylimidazole,2,4-dimethylimidazole, butylimidazole, 2-heptadecenyl-4-methylimidazole,2-methylimidazole, 2-undecenylimidazole, 1-vinyl-2-methylimidazole,2-n-heptadecylimidazole, 2-undecylimidazole, 2-heptadecylimidazole,2-phenylimidazole, 1-benzyl-2-methylimidazole,1-propyl-2-methylimidazole, 1-cyanoethyl-2-methylimidazole,1-cyanoethyl-2-ethyl-4-methylimidazole, 1-cyanoethyl-2-undecylimidazole,1-cyanoethyl-2-phenylimidazole, 1-guanaminoethyl-2-methylimidazole andaddition products of an imidazole methylimidazole and addition productsof an imidazole and trimellitic acid, 2-n-heptadecyl-4-methylimidazole,phenylimidazol, benzylimidazole, 2-methyl-4,5-diphenylimidazole,2,3,5-triphenylimidazole, 2-styrylimidazole, 1-(dodecylbenzyl)-2-methylimidazole,2-(2-hydroxyl-4-t-butylphenyl)-4,5-diphenylimidazole,2-(2-methoxyphenyl)-4,5-diphenylimidazole,2-(3-hydroxyphenyl)-4-,5-diphenylimidazole,2-(p-dimethyl-aminophenyl)-4,5-diphenylimidazole,2-(2-hydroxyphenyl)-4,5-diphenylimidazole,di(4,5-diphenyl-2-imidazole)-benzene-1,4,2-naphthyl-4,5-diphenylimidazole,1-benzyl-2-methylimidazole, 2-p-methoxystyrylimidazole, and thecombinations thereof.
 11. The composition according to claim 1, whereinthe imidazole component is 2-ethyl-4-methylimidazole.
 12. Athermosetting resin composition capable of sealing underfilling betweena semiconductor device and a circuit board to which said semiconductordevice is electrically connected, said composition comprising: (a) about92 parts by weight of an epoxy resin component comprising bisphenol Fepoxy resin, and (b) about 8 parts by weight of latent hardeningcomponent, of which 4 parts is comprised of (i) a cyanate estercomponent comprising 1,3-bis[4-cyanatophenyl-1-(methylethylidene)]benzene and 4 parts is comprisedof (ii) an imidazole component comprising 2-ethyl-4-methylimidazole. 13.A thermosetting resin composition capable of sealing underfillingbetween a semiconductor device and a circuit board to which saidsemiconductor device is electrically connected, said compositioncomprising: (a) about 88 parts by weight of an epoxy resin componentcomprising bisphenol F epoxy resin, and (b) about 12 parts by weight oflatent hardening component, of which 4 parts is comprised of a cyanateester component comprising1,3-bis[4-cyanatophenyl-1-(methylethylidene)]benzene and 8 parts iscomprised of an imidazole component comprising2-ethyl-4-methylimidazole.
 14. The composition of claim 1, having aviscosity of less than about 50,000 mPa.s at a temperature of 25° C. 15.Reaction products of a composition according to claim
 1. 16. A mountingstructure for semiconductor devices, comprising: a semiconductor devicecomprising a semiconductor chip mounted on a carrier substrate, and acircuit board to which said semiconductor device is electricallyconnected, wherein the space between the carrier substrate of saidsemiconductor device and said circuit board is sealed with a reactionproduct of a thermosetting resin composition according to claim
 1. 17. Athermosetting resin composition capable of sealing underfilling betweena semiconductor device and a circuit board to which said semiconductordevice is electrically connected, said composition consistingessentially of: (a) about 100 parts of an epoxy resin component; and (b)a latent hardener component in an amount up to about 15 parts, and (i) acyanate ester component in an amount up to about 15 parts, and (ii) animidazole component in an amount up to about 15 parts; and (c)optionally, a filler; (d) optionally, a defoaming agent; (e) optionally,a leveling agent; and (f) optionally, a dye.
 18. The mounting structureaccording to claim 16, wherein the thermosetting resin composition is inaccordance with claim
 17. 19. A process for fabricating semiconductordevices, said process comprising the steps of: electrically connecting asemiconductor device comprising a semiconductor chip mounted on acarrier substrate, to a circuit board; infiltrating a thermosettingresin composition into the space between the carrier substrate of saidsemiconductor device and said circuit board, wherein the thermosettingresin composition is in accordance with claim 1; and curing thethermosetting resin composition by the application of heat.
 20. Athermosetting resin composition having a gel time at a temperature ofabout 150° C. of about two minutes or less, and capable of sealingunderfilling between a semiconductor device and a circuit board to whichsaid semiconductor device is electrically connected, said compositioncomprising: (a) an epoxy resin component; and (b) a latent hardenercomponent comprising (i) a cyanate ester component, and (ii) animidazole component.
 21. A mounting structure for semiconductor devicescomprising: a semiconductor device comprising a semiconductor chipmounted on a carrier substrate, and a circuit board to which saidsemiconductor device is electrically connected, wherein the spacebetween the carrier substrate of said semiconductor device and saidcircuit board is sealed with a reaction product of a thermosetting resincomposition according to claim
 17. 22. A mounting structure forsemiconductor devices, comprising: a semiconnductor device comprising asemiconductor chip mounted on a carrier substrate, and a circuit boardto which said semiconductor device is electrically connected, whereinthe space between the carrier substrate of said semiconductor device andsaid circuit board is sealed with a reaction product of a thermosettingresin composition according to claim
 20. 23. The mounting structureaccording to claim 16, wherein the thermosetting resin composition is inaccordance with claim
 20. 24. The process according to claim 19, whereinthe thermosetting resin composition is in accordance with claim
 17. 25.The process according to claim 19, wherein the thermosetting resincomposition is in accordance with claim 20.